1. Field of the Invention
This invention relates generally to the combustion system of a gas turbine engine and, more particularly, to a double annular combustor having concentrically disposed inner and outer annular combustors with inner and outer domes, and a centerbody disposed between the inner and outer domes constructed of a plurality of annular segments.
2. Description of Related Art
Efforts to reduce emissions in gas turbine engines have brought about the use of staged combustion techniques wherein one burner or set of burners is used for low speed, low temperature conditions such as idle, and another, or additional, burner or burners are used for high temperature operating conditions. One particular configuration of such a concept is that of the double annular combustor wherein the two stages are located concentrically in a single combustor liner. Conventionally, the pilot stage section is located concentrically outside and operates under low temperature and low fuel/air ratio conditions during engine idle operation. The main stage section, which is located concentrically inside, is later fueled and cross-ignited from the pilot stage to operate at the high temperature and relatively high fuel/air ratio conditions. The swirl cups of the respective pilot and main stage sections generally lie in the same radial and circumferential planes, as exemplified by U.S. Pat. No. 4,292,801 to Wilkes, et al. and U.S. Pat. Nos. 4,374,466 and 4,249,373 to Sotheran.
However, as discussed in a development report to the National Aeronautics and Space Administration (NASA) on combustion system component technology for the Energy Efficient Engine (E.sup.3) and U.S. Pat. No. 4,194,358 to Stenger, the pilot stage and the main stage may be radially offset (i.e., lie in distinct radial planes). In both the '358 patent and E.sup.3 configurations, the effective length of the main stage section is relatively short and the effective length of the pilot stage section is relatively long. This configuration allows for complete or near-complete combustion to reduce the amount of hydrocarbon and carbon monoxide emissions since there is a relatively long residence time in the pilot stage section and a relatively minimal residence time in the main stage section.
Whether the inner and outer combustors are radially aligned or not, and whether the outer annular combustor acts as the pilot stage or main stage, the prior art discloses the use of a centerbody to isolate the pilot and main stages. The intended purpose of such centerbodies is to isolate the pilot stage from the main stage in order to ensure combustion stability of the pilot stage at various operating points and to allow primary dilution air to be directed into the pilot stage reaction zone.
To date, such centerbodies have been a continuous ring fabricated from forged or rolled rings and sheet material. This one-piece design is difficult to manufacture due to tight size and form tolerance requirements for fabrication and assembly. Further, the difference in temperature between the combustor structure and the centerbody generate large hoop stresses and associated forces at the point of attachment. This also occurs as a result of temperature differences in the individual members of the centerbody structure. Another problem with one-piece centerbodies is the effect on the entire piece caused by a local problem. For example, the entire centerbody is depressurized in the event of a local burn-through due to the resulting local leakage. Also, if one area of the centerbody is damaged the entire piece must be repaired or replaced. Accordingly, the present invention proposes an alternative centerbody design which eliminates the problems associated with one-piece centerbodies while maintaining the desirable characteristics thereof.